Second Generation Multiplex Plus
Second Generation Multiplex Plus (SGM Plus), is a DNA profiling system developed by Applied Biosystems. It is an updated version of Second Generation Multiplex. SGM Plus has been used by the UK National DNA Database since 1998.
An SGM Plus profile consists of a list of 10 number pairs, one number pair for each of 10 genetic markers, along with two letters (XX or XY) which show the result of the gender test. Each number pair denotes the two allele values for the marker - one value is inherited from each of the subject's parents. If both alleles are the same, then only a single number, rather than a pair, is recorded.
The genetic markers (or loci) used by SGM Plus are all Short tandem repeats (STRs). The markers used are: VWA, D8S1179, D21S11, D18S51, TH01, FGA, D3S1358, D16S539, D2S1338 and D19S433. Where a marker's designation begins with D, the digits immediately following the D indicate the chromosome that contains the marker. For example, D21S11 is on chromosome 21. SGM Plus also uses the Amelogenin (Amelo) sex-indicating test.
SGM Plus differs from SGM in that SGM does not use the markers D3S1358, D16S539, D2S1338 and D19S433.
SGM Plus has eight markers in common with CODIS FGA, TH01, VWA, D3S1358, D8S1179, D16S539, D18S51, and D21S11. It differs from CODIS in that it uses the additional markers D2S1338 and D19S433 and does not use the five markers CSF1PO, TPOX, D5S818, D7S820, D13S317.
|Common Sequence Motif||Allele
|FGA||4q28||(TTTC)3TTTT TTCT (CTTT)n CTCC (TTCC)2||12.2-51.2||215–353||NED|
|D3S1358||3p||TCTA (TCTG)1-3 (TCTA)n||8-21||114–142||5-FAM|
|D21S11||21q11.2–q21||(TCTA)n(TCTG)n[(TCTA)3TA(TCTA)3TCA (TCTA)2TCCA TA] (TCTA)n||12-41.2||187–243||JOE|
The primers are tagged with the following fluorescent dyes for detection under electrophoresis:
The primers for each locus are arranged on the dyes in the following order, from low molecular weight to large molecular weight:
- 5-FAM: D3, VWA, D16, D2
- JOE: Amelo, D8, D21, D18
- NED: D19, THO, FGA
The dyes to which each primer is attached differ from those of the original SGM DNA profiling system.
Example SGM Plus profile
An SGM Plus profile retrieved from a DNA database would just list the allele values:
15,18; 6,9; 11,13; 22,22; 31,32.2; 14,17; 17,20; 11,12; 13,16.3; 15,16; XY
Each value is the number of tandem repeats within the allele. A non-standard repeat is designated by the number of complete repeat units and the number of base pairs of the partial repeat, separated by a decimal point.
Probability of Identity
The probability of identify (also known as the random match probability) is the probability that two individuals selected at random will have an identical genetic profile.
Applied Biosystems estimates the probability of identity for SGM Plus to be approximately 1 in 13 trillion for African-Americans and 1 in 3.3 trillion Caucasian Americans.
The Human Genetics Commission has reported that the random match probability is in the region of 1 in a trillion. However it stated "When the SGM Plus profiling system was first introduced, there was agreement within the scientific community that identifications with match probabilities lower than one in a billion would not be quoted in the courts of law, so as to avoid overstating the value of the DNA evidence to take into account that match probabilities are only estimates, and to make sure that the figure used was one that was meaningful to non-specialists." 
The UK Crown Prosecution Service states "SGM Plus DNA profiling is very discriminating between individuals. The probability of obtaining a match between the profiles of two unrelated individuals by chance is very low, of the order of 1 in a billion [note this is a UK billion, ie a US trillion]. However, it has not yet been possible to carry out the required statistical testing to be able to quote this match probability, and in practice a more conservative chance match figure of 1 in 1,000 million is used."
- "For the Record" (PDF).
- AmpFlSTR SGM Plus PCR Amplification Kit User’s Manual (PDF). pp. 14–12.
- Human Genetics Commission. Nothing to hide, nothing to fear? p.49 (PDF) (Report).
- The Crown Prosecution Service. "B4. Adventitious (chance) DNA Matches".